Not just the heart but the entire being as well. That's why the Organization wears the cloaks. and the reaso nthat Saix and Xemnas were so close was kind of revealed in Days; that Saix and Axel planne dahead of time to become the second most powerful, and essentially become second in ocmmand for power.
well actually Saix planned to become 2nd in command....Axel was just an assassin really.